The present invention relates to an alloy for a sacrificial anode which is suitable for corrosion protection of reinforcement in a structure built of reinforced concrete and to a reinforced concrete structure comprising the sacrificial anode.
Reinforcement in a structure built of reinforced concrete is not substantially corroded because concrete is strongly resistant against alkali. However, the problem of corrosion arises when a reinforced concrete structure is in an environment where salt water may permeate therein. For example, such environments exist when the structure is near the sea or dusted over by chlorides for the prevention of ice accumulation.
Most cathodic protection of steel in concrete is done with impressed current systems. Impressed current systems have the inherent need for periodic maintenance which limits their attractiveness to bridge owners. However, the application of impressed current anodes requires that the anode be completely isolated from the embedded steel, otherwise short circuits will occur. Sacrificial anode systems do not have these problems.
In an attempt to solve the above-noted problem, use of a zinc alloy has been proposed in a sacrificial anode method which realizes long-term, stable and low-cost corrosion protection. However, a sacrificial anode formed of a zinc alloy has an exceedingly high potential (high positive). A low potential (high negative potential) is one of the important characteristics of a sacrificial anode.
Furthermore, pure zinc, aluminum, and aluminum-zinc alloys have been used for sacrificial cathodic protection of steel reinforcing in concrete. All of these alloys have exhibited a phenomenon called passivation while on concrete. Passivation occurs when the pH of the concrete surface decreases below the normally highly alkaline value found in concrete as a result of reactions with carbon dioxide in the air, a process called carbonation, which is a normal process. The effect of passivation is that the current output of the alloy anode decreases to a point which is no longer satisfactory to provide cathodic protection for the steel. These alloys are only satisfactory for use in very wet areas of the structure.
The alloys of the present invention do not exhibit the above-identified passivation phenomenon and maintain a satisfactory level of cathodic protection current. Accordingly, the present invention provides an alloy for a sacrificial anode which is suitable for corrosion protection of reinforcement in a structure built of reinforced concrete; namely, an alloy which enables a sacrificial anode formed thereof to have a sufficiently low potential and to cause generation of a sufficiently large amount of electricity.
An alloy for a sacrificial anode according to a first preferred aspect of the present invention includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, and about 0.0005% to about 0.05% of Zr. The balance may be Al and any unavoidable impurities. An alloy according to a second preferred aspect of the present application includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, and about 0.05% to about 0.3% of Si. The balance may be Al and any unavoidable impurities. An alloy according to a third preferred aspect of the present invention includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, and about 0.02% to about 0.2% of Ce. The balance may be Al and any unavoidable impurities. An alloy according to a fourth preferred aspect of the present invention includes about 10% to about 50% of Zn, about 0.03% to about 0.6% of In, about 0.005% to about 0.1% of Ti, and about 0.001% to about 0.02% of B. The balance may be Al and any unavoidable impurities. An alloy according to another preferred aspect of the present invention includes about 10% to about 50% of Zn and about 0.03% to about 0.6% of In. The balance may be Al and any unavoidable impurities.
The present invention also relates to a reinforced concrete structure comprising a cementitious material, metal reinforcement, and a sacrificial anode, the sacrificial anode including an alloy containing Al, Zn and In. The alloy may further contain one or more of Zr, Si, Ce, Ti and B.
The present invention further relates to a method of providing cathodic protection to a reinforced concrete structure comprising providing a reinforced concrete structure comprising a cementitious material and metal reinforcement; and introducing a cathodic protection anode into the reinforced concrete structure, the anode including an alloy comprising Al, Zn and In. The method may further comprise electrically connecting the sacrificial anode to the metal reinforcement. The alloy may further contain one or more of Zr, Si, Ce, Ti and B.
The present invention also relates to a method of making a cathodically protected reinforced concrete structure comprising providing a reinforced concrete structure comprising a cementitious material and metal reinforcement; introducing a sacrificial anode into the reinforced concrete structure and electrically connecting the sacrificial anode to the metal reinforcement. The sacrificial anode includes an alloy containing Al, Zn and In, and may further contain one or more of Zr, Si, Ce, Ti and B.
Unless otherwise specified herein, in this specification and in the appended claims all amounts indicated are percent by weight.
In an alloy according to the present invention, both Zn and In function so as to restrict self dissolution of the alloy thus increasing the amount of electricity generated. In a preferred embodiment, if the amount of Zn contained in the alloy is less than about 10%, or if the amount of In contained in the alloy is less than about 0.03%, the above-described function is not sufficiently effected. Also, if the amount of Zn contained in the alloy is more than about 50%, or if the amount of In contained in the alloy is more than about 0.6%, the potential of the anode tends to be too high (too highly positive). In a more preferred embodiment, the amount of Zn contained in the alloy is about 10% to about 40%. In another more preferred embodiment, the amount of Zn is about 10% to about 30%. In a more preferred embodiment, the amount of In contained in the alloy is about 0.05% to about 0.5%. In another more preferred embodiment, the amount of In is about 0.1% to about 0.3%.
In an alloy according to the first preferred aspect of the invention, Zr has the same function as Zn and In. In a preferred embodiment, if the amount of Zr contained in the alloy is less than about 0.0005%, the function of restricting self dissolution is not sufficiently effected. Also, if the amount of Zr contained in the alloy is more than about 0.05%, Zr is distributed in the grain boundary of the alloy in large grains thus reducing the amount of electricity generated. In a more preferred embodiment, the amount of Zr contained in the alloy is about 0.001% to about 0.01%.
In an alloy according to a second preferred aspect of the invention, Si has the same function as Zn and In. In a preferred embodiment, if the amount of Si contained in the alloy is less than about 0.05%, the function of restricting self dissolution is not sufficiently effected. Also, if the amount of Si contained in the alloy is more than about 0.3%, the potential of the anode formed thereof tends to be too high (too highly positive). In a more preferred embodiment, the amount of Si contained in the alloy is about 0.1% to about 0.2%.
In an alloy according to a third preferred aspect of the invention, Ce functions so as to prevent hole-type corrosion of the alloy thus increasing the amount of electricity generated. In a preferred embodiment, if the amount of Ce contained in the alloy is less than about 0.02%, the function is not sufficiently effected. Also, if the amount of Ce contained in the alloy is more than about 0.2%, the potential of the anode formed thereof tends to be too high (too highly positive). In a more preferred embodiment, the amount of Ce contained in the alloy is about 0.05% to about 0.1 5%.
In an alloy according to a fourth preferred aspect of the invention, both Ti and B function so as to prevent hole-type corrosion and groove-type corrosion (corrosion occurring in the form of a groove leaving two sides of the groove uncorroded) of the alloy by making the crystals of the alloy microscopic grains instead of large pillars thus increasing the amount of electricity generated. In a preferred embodiment, if the amount of Ti contained in the alloy is less than about 0.005%, or if the amount of B contained in the alloy is less than about 0.001%, the function is not sufficiently effected. Also, if the amount of Ti contained in the alloy is more than about 0.1%, or if the amount of B contained in the alloy is more than about 0.02%, the amount of electricity generated is reduced. In a more preferred embodiment, the amount of Ti contained in the alloy is about 0.01% to about 0.08%. In another more preferred embodiment, the amount of B is about 0.005% to about 0.01%.